This invention relates to novel polyimides having high solubility in organic solvents and excellent low-temperature moldability, a process for preparing such polyimides, and thermosetting resin compositions containing said polyimides, showing excellent properties even after low-temperature curing and useful as an adhesive, laminating material, molding material and the like.
Polyimide resins have excellent heat resistance, but many of them are poor in moldability because of insufficient solubility and fusibility. Therefore, for molding a film or other products from a polyimide, it has been necessary to perform molding by using a varnish prepared by dissolving a polyamide acid, which is a precursor of polyimide, in an organic solvent, and then conduct an imidation treatment on the molded product. However, a varnish comprising a polyamide acid is lowered in molecular weight when hydrolyzed, so that it must be kept at a low temperature. The above-said prior art molding process also had problems such as precipitation of the resin due to moisture absorption since the solvents usually used in said process are hydroscopic, formation and retention of voids in the molded product due to generation of an aqueous condensate in the course of imidation after molding of a desired product such as film, and unapplicability to a base material with poor heat resistance since use of a high temperature above 300.degree. C. is required for the imidation treatment.
Various polyimides having a softening point and therefore capable of molding have been developed and proposed for solving said prior art problems.
For instance, Japanese Patent Unexamined Publication No. 62-10051 discloses a polyimide composed of a diamine and pyromellitic acid, which is represented by the formula: ##STR1## wherein X' stands for C.dbd.O or SO.sub.2. This polyimide has a softening point and is capable of molding, but because of its too high softening point, molding must be carried out at a temperature over 300.degree. C. Also, this polyimide is scarcely or only slightly soluble in organic solvents, so that for molding a thin film therefrom, it is necessary to first form a film by using a varnish prepared by dissolving a polyamide acid, which is a precursor of said polyimide, in an organic solvent and then imidize the film compound by heating at a high temperature. Thus, this polyimide still involves the problems such as mentioned above.
The polyimides soluble in organic solvents have also been developed for overcoming said problems. For example, the polyimides composed of 4,4'-methylenebis(2,6-dialkylanyline) and acid dianhydrides such as biphenyltetracarboxylic acid dianhydride, pyromellitic acid dianhydride, etc., disclosed in U.S. Pat. No. 4,851,505 (Japanese Patent Unexamined Publication No. 1-263116) and U.S. Pat. No. 4,935,490 (Japanese Patent Unexamined Publication No. 1-263117) are soluble in N-methylpyrrolidone and m-cresol. These polyimides, however, have as high a glass transition point as over 400.degree. C. and require a high temperature for molding.
Also, Japanese Patent Examined Publication No. 43-19814 (German Offenlegungsschrift No. 1,520,012) discloses polyimides obtained from paraphenylene-bis(trimellitate) dianhydrides and diamines such as benzidine, 4,4'-diaminodiphenyl ether, etc., and Japanese Patent Examined Publication No. 43-5911 teaches that bisphenol A bistrimellitate dianhydrides can be used as a starting material for the preparation of polyimides (no example of preparation of polyimide by using said substance is shown). Any of the polyimides disclosed or mentioned in these patent publications is higher than 350.degree. C. in softening point, and no teaching is made on their solubility in the organic solvents with small polarity.
U.S. Pat. No. 4,681,928 (Japanese Patent Unexamined Publication No. 60-258225) discloses the polyimides obtainable by reacting 1,4-bis(p-aminocumyl)benzene and a dianhydride of an aromatic or aliphatic acid, and it is verified that such polyimides are soluble in N-methylpyrrolidone. It is also taught that these polyimides are fusible (the glass transition point being around 200.degree. C.). 2,2-bis[4,4-(3,4-dicarboxyphenoxy)phenyl]propane, etc., are mentioned as examples of said acid dianhydride. Paraphenylene-bis(trimellitate)dianhydride is also referred to as an example of said acid dianhydride, but there is shown no example of preparation of polyimide by using said compound. These polyimides are insoluble in toluene and ethylene glycol dimethyl ether which are smaller in polarity than N-methylpyrrolidone. No mentioned is made of the solubility of said polyimides in the organic solvents with small polarity, such as dioxane, tetrahydrofuran and methylene chloride.
Further, Japanese Patent Unexamined Publication No. 2-11633 discloses a process for preparing a polyimide by reacting a diamine having 6-member rings in the main chain, such as the diamine of the afore-shown formula A, with a specific acid dianhydride (e.g. bisphenol A bistrimellitate dianhydride). The polyimides prepared according to this process are low in softening point and also improved in solubility in organic solvents in comparison with the polyimides disclosed in Japanese Patent Unexamined Publication No. 62-10051. Regarding solubility in organic solvents, the polyimides obtained according to the process disclosed in Japanese Patent Unexamined Publication No. 2-11633 are soluble in N,N-dimethylformamide, dioxane, etc., but insoluble in toluene and diethylene glycol dimethyl ether. Also, said diamines are not only difficult to produce with high purity but also expensive, so that their industrial use is disadvantageous in terms of economy.
As noted from the foregoing, it has been very difficult to produce the polyimides which are low in softening point and have sufficiently high solubility in organic solvents.
On the other hand, the resin compositions prepared by adding polymaleimides to polyamide acids or polyimides have a wide scope of use as the cured products of said resin compositions are tough and high in heat resistance. Said resin compositions are used, for instance, for the preparation of adhesives, varnish for printed wiring boards, and prepregs. Prepregs can be used for producing the printed wiring boards, etc.
U.S. Pat. No. 4,362,826 discloses a thermosetting resin composition comprising a polyamide acid obtained from 4,4'-diaminodiphenyl ether and pyromellitic acid dianhydride and a bismaleimide compound. It is shown that this composition can be worked into a cured film (film supported on a substrate) by cast coating said composition onto a substrate such as a copper foil and drying the coat with heating.
However, the resin compositions prepared by adding bismaleimides to polyamide acids such as mentioned above are required, when cured, to not only react a polymaleimide but also imidate the polyamide acid. This necessitates heating at a temperature above 300.degree. C. for curing, so that these compositions can not be applied to a base material with low heat resistance. Also, since an aqueous condensate is generated concomitant with imidation of the polyamide acid, formation of the voids is unnegligible although it is suppressed to some extent by the presence of polymaleimide. Especially, when said composition is interposed between a Kapton film (polyimide film) and a metal foil such as copper foil and cured for bonding them, formation of the voids causes an excessive reduction of adhesive force.
It is well known in the art that the polyimides obtained by imidating the amide acids such as disclosed in U.S. Pat. No. 4,362,826 are insoluble in organic solvents and also infusible. Further, no example of polyimides soluble in organic solvents is shown in this U.S. patent.
Regarding the compositions containing a polyimide and a polymaleimide, U.S. Pat. No. 3,842,143 discloses a composition prepared by adding a polymaleimide to an amino-terminated low-molecular weight polyimide obtained by reacting a tetracarboxylic acid dianhydride with an excess of a diamine. However, in case of using such a low-molecular weight polyimide, the film obtained from said composition lacks flexibility,and no self-supporting film can be obtained. In this U.S. patent, no mention is made of a composition having enough flexibility to form a self-supporting film nor is shown any polyimide to be used for such a composition.
U.S. Pat. No. 4,985,509 (Japanese Patent Application Unexamined Publication No. 62-30112) discloses a composition containing a polyimide obtained from 3,3',4,4'-benzophenontetracarboxylic acid dianhydride, bis[4-(3-aminophenoxy)phenyl]sulfone and if necessary 2,4-diaminotoluene and a polymaleimide and states that this composition has good film-forming properties and the film made therefrom shows excellent adhesiveness. It is also taught that said polyimide is soluble in a toluene/m-cresol mixed solvent. However, the polyimide disclosed in this U.S. patent is insoluble in the organic solvents with small polarity such as tetrahydrofuran, dioxane and methylene chloride. Also, the composition disclosed therein is fusible and cured at a relatively low temperature (250.degree.-275.degree. C.) and shows excellent adhesiveness, but this composition is deteriorated in its properties when cured at a temperature lower than 230.degree. C.
In the afore-mentioned Japanese Patent Unexamined Publication No. 62-10051, U.S. Pat. Nos. 4,851,505, 4,935,490, Japanese Patent Examined Publication No. 43-18914 (DE-OS 1,520,012), Japanese Patent Examined Publication No. 43-5911, U.S. Pat. No. 4,681,928 and Japanese Patent Unexamined Publication No. 2-11633, no disclosure is made of the compositions containing a polyimide and a polymaleimide.
As understood form the above, there is yet known no composition which contains a polyimide and a polymaleimide and which shows the excellent characteristics when cured at a satisfactorily low temperature and also has good film-forming properties. Nor is known a polyimide suited for use as a component of such a composition.